The storage system connected to the host computer via a network has multiple magnetic disks, for example, as storage devices for storing data. In the storage system, RAID (Redundant Array of Independent Disks) groups (also referred to as parity groups) are constituted by forming redundant storage areas composed of multiple storage devices via RAID technique. Then, the storage system provides a portion of the RAID groups as a storage area having a capacity required by the host computer in the form of a logical volume to the host computer.
Further, a technique is disclosed that gathers multiple physical storage systems and provides the same as a single virtual storage system to the host computer (refer for example to Patent Literature 1). This technique enables to manage multiple physical storage systems as a single storage system.